Wouldn't that incentive people to just return all black images and render nothing to get free points?
Seriously though, any platform is going to have its share of liars, cheaters, grifters, con artists, and outright scams. What the world relies on is that enough people aren't. The trick is attracting enough honest people while driving away enough of the dishonest people. it's a constant game of cat and mouse.
Better yet, make all abusers render each other frames.